PHILADELPHIA (CBS) – NASA still isn’t sure where a defunct spacecraft will fall as it heads toward Earth.

The Upper Atmosphere Research Satellite (UARS) weighs six tons and as big as a school bus, but most of it will break up and burn before hitting the earth’s atmosphere, according to Derrick Pitts, chief astronomer at the Franklin Institute in Philadelphia.

Pitts says there are about 26 pieces of the satellite with a total weight of 1,200 pounds that have a chance of reaching the surface of the earth.

And Pitts points out that since the earth is 75 percent covered with water, the likelihood is that it will fall in an ocean rather than on land.

“Even if this comes down over land,” he adds, “when you look at how the population of the planet is distributed, most of the earth’s surface is pretty much empty anyway, so that again reduces the chances of someone being injured by falling pieces of debris. That has never happened before.”

He says there will be a better indication about midday Friday where the satellite could fall.